University of Louisville ThinkIR: The University of Louisville's Institutional Repository Faculty Scholarship 2-2014 The ultraviolet attenuation law in backlit spiral galaxies. William C. Keel University of Alabama - Tuscaloosa Anna M. Manning Stennis Space Center Benne W. Holwerda University of Louisville Chris J. Lintott Oxford University Kevin Schawinski ETH Zurich Follow this and additional works at: https://ir.library.louisville.edu/faculty Part of the Astrophysics and Astronomy Commons Original Publication Information Keel, William C., et al. "The Ultraviolet Attenuation Law in Backlit Spiral Galaxies." 2014. The Astronomical Journal 147(2): 14 pp. This Article is brought to you for free and open access by ThinkIR: The University of Louisville's Institutional Repository. It has been accepted for inclusion in Faculty Scholarship by an authorized administrator of ThinkIR: The University of Louisville's Institutional Repository. For more information, please contact [email protected]. The Astronomical Journal, 147:44 (14pp), 2014 February doi:10.1088/0004-6256/147/2/44 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE ULTRAVIOLET ATTENUATION LAW IN BACKLIT SPIRAL GALAXIES∗ William C. Keel1,8,9, Anna M. Manning2,8, Benne W. Holwerda3,4, Chris J. Lintott5,6,8, and Kevin Schawinski7 1 Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487, USA; [email protected], Twitter@NGC 3314 2 Stennis Space Center, MS 39522; [email protected] 3 ESA-ESTEC, Keplerlaan 1, 2201-AZ Noordwijk, The Netherlands; [email protected], Twitter@BenneHolwerda 4 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands 5 Astrophysics, Oxford University, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 6 Adler Planetarium, 1300 S. Lakeshore Drive, Chicago, IL 60605, USA; [email protected], Twitter@chrislintott 7 Institute for Astronomy, ETH Zurich,¨ Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland; [email protected], Twitter@kevinschawinski Received 2013 September 6; accepted 2013 December 28; published 2014 January 14 ABSTRACT The effective extinction law (attenuation behavior) in galaxies in the emitted ultraviolet (UV) regime is well known only for actively star-forming objects and combines effects of the grain properties, fine structure in the dust distribution, and relative distributions of stars and dust. We use Galaxy Evolution Explorer, XMM Optical Monitor, and Hubble Space Telescope (HST) data to explore the UV attenuation in the outer parts of spiral disks which are backlit by other UV-bright galaxies, starting with the candidate list of pairs provided by Galaxy Zoo participants. New optical images help to constrain the geometry and structure of the target galaxies. Our analysis incorporates galaxy symmetry, using non-overlapping regions of each galaxy to derive error estimates on the attenuation measurements. The entire sample has an attenuation law across the optical and UV that is close to the Calzetti et al. form; the UV slope for the overall sample is substantially shallower than found by Wild et al., which is a reasonable match to the more distant galaxies in our sample but not to the weighted combination including NGC 2207. The nearby, bright spiral NGC 2207 alone gives an accuracy almost equal to the rest of our sample, and its outer arms have a very low level of foreground starlight. Thus, this widespread, fairly “gray” law can be produced from the distribution of dust alone, without a necessary contribution from differential escape of stars from dense clouds. Our results indicate that the extrapolation needed to compare attenuation between backlit galaxies at moderate redshifts from HST data, and local systems from Sloan Digital Sky Survey and similar data, is mild enough to allow the use of galaxy overlaps to trace the cosmic history of dust in galaxies. For NGC 2207, HST data in the near-UV F336W band show that the covering factor of clouds with small optical attenuation becomes a dominant factor farther into the UV, which opens the possibility that widespread diffuse dust dominates over dust in star-forming regions deep into the UV. Comparison with published radiative-transfer models indicates that the role of dust clumping dominates over differences in grain populations at this coarse spatial resolution. Key words: galaxies: ISM – galaxies: spiral – ultraviolet: galaxies 1. INTRODUCTION UV and optical attenuation measures. Existing models have a wide range in the predicted behavior of both dust mass Our understanding of the effects of dust grains in galaxies has and resulting attenuation with redshift (Calzetti & Heckman increased dramatically with such new capabilities as sensitive 1999), since dust production, destruction, and the shrinking far-infrared (FIR) measures, spatially resolved modeling of the mass fraction in the interstellar medium (ISM) compete in ways spectral-energy distributions (SEDs) of star/grain mixes, and that are not well constrained at large redshifts. Broadly, at- photometry of resolved galaxies giving independent reddening tenuation from SED fits and photometric redshifts has shown maps from the stars themselves (Berry et al. 2012; Dalcanton a peak at z ≈ 1.5, declining at earlier and later epochs et al. 2012). The emerging starlight is modified in ways which (Rowan-Robinson 2003). Observationally, fits to SEDs from depend crucially on the relative distributions of stars and dust, the UV to FIR by Iglesias-Paramo´ et al. (2007) indicate that the and on the small-scale structure in the dust. dust content of low-mass galaxies has increased from z = 0.7 These factors produce proportionally greater uncertainties to the present, while galaxies at high stellar mass show no such in the emitted ultraviolet (UV) range, affecting much of the trend. Deep surveys over wide spectral ranges are now deep and data relevant to galaxy evolution. As extensive Hubble Space wide enough to test models for the evolution of dust. Connec- Telescope (HST) surveys are allowing exploration of the evo- tion of the history of dust mass to observables must fold in at lution of galaxy morphology, they can also help track the least implicit knowledge of the dust distribution on both large evolution of the dust content of galaxies if we can connect and small scales. ∗ A result that has found wide applicability is the effective Based in part on observations made with the NASA Galaxy Evolution extinction law derived by Calzetti et al. (1994), based on Explorer. GALEX is operated for NASA by the California Institute of Technology under NASA contract NAS5-98034. the SEDs of star-forming galaxies. It is relatively flat with 8 Visiting astronomer, Kitt Peak National Observatory, National Optical wavelength as compared to the behavior found from star-by- Astronomy Observatories, which is operated by the Association of Universities star studies, implying that it is strongly affected by the relative for Research in Astronomy, Inc. (AURA) under cooperative agreement with distributions of stars and dust and by unresolved fine structure the National Science Foundation. The WIYN Observatory is a joint facility of the University of Wisconsin–Madison, Indiana University, Yale University, in the dust distribution itself. The UV range is particularly and the National Optical Astronomy Observatory. sensitive to these effects, due to the short lifetimes of the stars 9 SARA Observatory. that dominate the UV light from star-forming systems, and to 1 The Astronomical Journal, 147:44 (14pp), 2014 February Keel et al. the inevitable bias in favor of more transparent areas within a make it a useful complement to, for example, FIR and submil- finite region of a galaxy (Fischera et al. 2003). Keel & White limeter survey results. (2001) found that that measured reddening behavior in two For most of our studies of dust in backlit galaxies, we backlit spiral galaxies becomes flatter (grayer) when the data have fairly strict symmetry requirements, so we can trace are smoothed over successively larger regions before analysis. radial behavior. For this project, since we are most interested Effects of dust structure must be included in SED models in in the wavelength behavior of attenuation, we can relax this order to retrieve either the intrinsic stellar SED or the effective requirement as long as we can quantify the effects of galaxy extinction. Comparison of galaxy disks seen at various angles asymmetry on our derived attenuation values. Thus, our sample can provide independent information on some of the distribution here includes some galaxies showing mild effects of interactions issues (i.e., Wild et al. 2011). (most notably NGC 2207/IC 2163). This issue highlights the distinction between the observable In this paper, we report an extension of backlighting mea- quantity attenuation, measured on some size scale, and the actual surements into the UV, where most previous attenuation re- extinction attributable to the grain properties, which is typically sults within galaxies are limited either to very nearby systems manifested in what are essentially point-source measurements of (Bianchi et al. 1996; Dalcanton et al. 2012) or to actively star- light from individual stars, where mixture and scattering effects forming galaxies (Calzetti et al. 1994). One motivation for this are negligible (Witt & Gordon 2000). In interpreting observa- study is the promise of using similar techniques in deep HST tions of galaxies, the net loss of direct starlight is often